This invention relates to high power electronic components, such as resistors, and is more particularly concerned with an assembly comprising the component and a bushing on which it is mounted that couples the component thermally to a conducting surface, such as a transistor flange mounted onto a heat sink.
In RF power systems, such as transistorized amplifiers and RF frequency generators, it is often required to dissipate large amounts of power through a feedback resistor, resistive termination, or other suitable device.
At the present time, many high powered radio frequency amplifiers employ high power feed back resistors associated with their power transistors. These are of two basic types, Parallel and Series Feedback resistors. Parallel Feedback resistors are typically 25 to 100 ohms in value and dissipate power up to 25-50 watts. Consequently, reliability considerations require power dissipation ratings up to 100 watts or more. Series Feedback resistors are typically 0.5 to 5 ohms in value and require similar high power dissipation levels and associated high power ratings. Ceramic resistors are often employed for this purpose. Ceramic resistors rely on convective air cooling. These resistors are typically very large, and even then the largest are only rated at about ten watts. Thus, the ceramic power resistors can be over driven, and in operation can become quite hot, often red-hot. At elevated temperatures, the resistive value changes, which, in turn, changes the gain characteristic and the impedance of the amplifier. This, in turn, can destabilize the amplifier, resulting in RF instability and spurious output and even catastrophic destruction of the transistor.
It has been proposed to employ flanged or "half-flange" resistors of the type in which a flat film resistor or other device is formed on a flat metal plate, with one end of the plate serving as both a carrier for the resistor or other device and a heat spreader type mounting base. This type of device can be configured similar to a TO-220 device. In this configuration the flat metal flange is in the same plane as the flat resistive or other type of circuit element, and the flange lies secured to the chassis or other suitable heat sink through a hole in the flange. The problem for devices of this configuration is that they are both geometrically incorrect and physically too large for mounting on a transistor's cooling flange. Thus, if half-flange or "full" flange resistors are employed, it is necessary to redesign and reconfigure the circuit board or chassis to accommodate them. This can involve completely revising the circuit architecture, which is already quite dense and very compact and may even be prohibited altogether by size constraints.